Tracking of handheld sporting implements using computer vision

ABSTRACT

A path and/or orientation of object approaching an athlete is tracked using two or more cameras. At least two sets of images of the object are obtained using at least two different cameras having different positions. Motion regions within images are identified, and candidate locations in 2D space of the object are identified within the motion region(s). Based thereon, a probable location in 3D space of the identifiable portion is identified, for each of a plurality of instants during which the object was approaching. A piecewise 3D trajectory of at least the identifiable portion of the object is approximated from the probable locations in 3D space of the object for multiple instants during which the object was approaching the athlete. A graphical representation of the 3D trajectory of the object is incorporated into at least one of the sets of images.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to and claims priority from the following USpatent application. This application is a continuation of U.S. patentapplication Ser. No. 15/845,523 filed Dec. 18, 2017, which is acontinuation of U.S. non-provisional application Ser. No. 15/072,176filed Mar. 16, 2016, each of which is incorporated herein by referencein their entirety.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to tracking of handheld sportingimplements using computer vision.

2. Description of the Prior Art

Many sports involve an athlete swinging a handheld sporting implement inan attempt to strike another object. Such a handheld sporting implementis often a long, stick-like object, such as a baseball bat, a cricketbat, a golf club or a hockey stick, which is swung in an attempt to hita ball or a puck. The technique and precision with which the athleteperforms this swinging motion directly affects the athlete'sperformance, as well as the performance of an entire team of athletes,in the case of team sports. The present boom in sports analyticsprovides a strong demand for scrutinizing an athlete's swingingtechnique in order to take the athlete's performance to increasinglyhigher-skilled levels.

SUMMARY OF THE INVENTION

The present invention relates to systems and methods for tracking anobject approaching an athlete.

Embodiments described herein can be used for tracking a path and/ororientation of at least a portion of a handheld sporting implement thatis swung by an athlete. The handheld sporting implement, which can be,e.g., a baseball bat, a cricket bat, a golf club, or a hockey stick, mayhave a shaft extending between two ends, such as a head and a knob. Amethod according to an embodiment of the present technology includesreceiving two or more different sets of video images of a handheldsporting implement being swung by an athlete, wherein at least two ofthe different sets of video images are captured using at least twodifferent cameras having different positions. The method also includesidentifying one or more motion regions within each of a plurality of thevideo images in each of at least two of the different sets of videoimages. One or more candidate locations in two-dimensional (2D) space ofan identifiable portion (e.g., the head) of the handheld sportingimplement is/are identified within the identified motion region(s) ofthe video image, for at least a subset of the video images included inat least two of the different sets of video images. Based on thecandidate locations in 2D space of the identifiable portion (e.g., thehead) of the handheld sporting implement, a probable location inthree-dimensional (3D) space of the identifiable portion (e.g., thehead) of the handheld sporting implement is identified, for each of aplurality of instants during which the handheld sporting implement wasswung by the athlete. Additionally, a piecewise 3D trajectory of atleast the identifiable portion (e.g., the head) of the handheld sportingimplement is approximated from the probable locations in 3D space of theidentifiable portion (e.g., the head) of the handheld sporting implementidentified for the plurality of instants during which the handheldsporting implement was swung by the athlete. Such embodiments can beextended to track the path of more than just the head of the handheldsporting implement during a swing, and more specifically, can beextended to track the path of the entire shaft of the swung handheldsporting implement.

This Summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This Summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used as an aid in determining the scope of the claimed subjectmatter.

These and other aspects of the present invention will become apparent tothose skilled in the art after a reading of the following description ofthe preferred embodiment when considered with the drawings, as theysupport the claimed invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A depicts a baseball park and equipment for obtaining video imagesof an athlete swinging a baseball bat type of handheld sportingimplement.

FIG. 1B depicts further details of the processing facility and camerasof FIG. 1A, and more generally, depicts a computer system for tracking apath of a handheld sporting implement that is swung by an athlete.

FIG. 1C depicts a relationship between camera, image and worldcoordinate systems, for use by the processing facility of FIG. 1A for,among other things, tracking a path of a handheld sporting implementthat is swung by an athlete.

FIG. 2 is a high level flow diagram that is used to describe certainmethods for tracking a path and/or orientation of at least a portion ofa handheld sporting implement that is swung by an athlete.

FIG. 3A illustrates an exemplary video image (also known as a videoframe) obtained by one of the cameras shown in FIG. 1A.

FIG. 3B illustrates motion regions identified by comparing the videoimage in FIG. 3A to a preceding video image and/or a following videoimage in a sequence of the video images captured by the same one of thecameras in FIG. 1A.

FIG. 3C illustrates outlines of the motion regions shown in FIG. 3B, aswell as candidate shafts of the handheld sporting implement identifiedwithin the motion regions, and candidate heads of the handheld sportingimplement.

FIG. 4 is a high level flow diagram that is used to describe additionaldetails of how motion regions within a video image can be identified atstep 204 initially discussed with reference to FIG. 2.

FIG. 5 is a high level flow diagram that is used to describe additionaldetails of how candidate locations in 2D space of a head of a handheldsporting implement can be identified at step 206 initially discussedwith reference to FIG. 2.

FIG. 6 includes dots that represent candidate heads of a handheldsporting implement determined from a plurality of video images capturedusing a same camera while an athlete swung the handheld sportingimplement.

FIG. 7 is a high level flow diagram that is used, together with FIG. 6,to describe additional details of how to identify an approximate centerof elliptical arc motion of a swung handheld sporting implement at step504 initially discussed with reference to FIG. 5.

FIG. 8 is a high level flow diagram that is used to describe additionaldetails of how to identify candidate location(s) in 2D space of a headof a handheld sporting implement within identified motion region(s) of avideo image at step 506 initially discussed with reference to FIG. 5.

FIG. 9 includes dots and an elliptical arc that are superimposed on avideo image of an athlete holding a bat during the start of a swingingmotion, wherein the superimposed dots are illustrative of candidatelocations in 2D space of the head of the bat that were identified for aplurality of video images that followed the video image shown in FIG. 9,after false positives were filtered out, and wherein the superimposedelliptical arc is illustrative of an estimated trajectory of the head ofthe bat during the swing.

FIG. 10 is a high level flow diagram that is used to describe additionaldetails of how to identify a probable location in 3D space of a head ofa handheld sporting implement at step 208 initially introduced withreference to FIG. 2.

FIG. 11 shows lines of position from three cameras, wherein each line ofposition represents an imaginary straight line that extends from acamera to a tracked object (the head of a handheld sporting implement,in this example) at a given point in time, and identifies a locus ofpoints at which the object could be located based on the camera'sobservation.

FIG. 12 illustrates an approximated piecewise 3D trajectory of the headof a handheld sporting implement that is based on identified probablelocations in 3D space of the head of the handheld sporting implement.

FIGS. 13A and 13B illustrates candidate locations of a bat head andprobable locations of the bat head, as determined from the candidatelocations, after they had been projected from 3D space back into 2Dspace and superimposed on video images of the handheld sportingimplement being swung.

FIG. 14 is used to illustrate how the path of an entire shaft of ahandheld sporting implement that is swung by an athlete can be tracked,in accordance with certain embodiments.

DETAILED DESCRIPTION

The present invention is generally directed to systems and methods fortracking objects approaching athletes.

None of the prior art discloses a system that uses two or more camerasto approximate the trajectory of an object approaching a player andincorporate a graphical representation of the 3D trajectory of theobject into video from one of the cameras.

Referring now to the drawings in general, the illustrations are for thepurpose of describing one or more preferred embodiments of the inventionand are not intended to limit the invention thereto.

Embodiments of the present technology can be used to track a path and/ororientation of at least a portion of a handheld sporting implement thatis swung by an athlete. Such embodiments can provide a fully- orsemi-automated computation of a three-dimensional path of an athlete'sswing. This trajectory of motion can be used to aid an athlete in avariety of ways. For example, a swing can be correlated with the outcomeof the swing, enabling an athlete or other person (or a system) tocompare desired outcomes with the swings that produced them, andultimately, to fine-tune the athlete's swing to produce those desiredoutcomes. For more specific examples, an athlete may attempt to adjustthe arc of his or her swing in order to match the one which produced ahole-in-one in golf, a 100 mile per hour (M.P.H.) slap shot in hockey,or a home run in baseball.

Professional sports have become a large business in the United States,putting increasing pressure on sports team owners to make wise“investments” in the players they choose for their teams. The analysisof an athlete's swing can aid these executives, as well as coaches andtrainers, in the evaluation of prospective athletes, providinginformative, objective measures of the athlete's talent.

Further, success of both an individual athlete and a team of athletesdepends largely on the health of those involved. A single injury can endan athlete's season or career, can be fatal to a team's performance, andcan potentially lead to financial ruin for the sports club or individualinvolved. Embodiments of the present technology enable the evaluation ofsubtleties in an athlete's swinging motion that may cause an injury, andin turn, can be used to avoid such motions to keep an athleteinjury-free.

For certain embodiments, it is assumed herein that the handheld sportingimplement that is swung by an athlete includes a shaft extending betweena head and a knob. For example, the handheld sporting implement can be abaseball bat or a cricket bat that includes a bat head at one end of abat shaft and a bat knob at the other end of the shaft close to where anathlete grips the bat. For another example, the handheld sportingimplement can be a golf club that includes a club head at one end of ashaft and a knob at the end of the shaft close to where an athlete gripsthe golf club. For still another example, the handheld sportingimplement can be a hockey stick that includes a head that is typicallyreferred to as a hockey blade at one end of the shaft and a knob at theother end of the shaft close to where an athlete grips the hockey stick.The head of the handheld sporting implement, as the term is used herein,refers to the distal end of the shaft of the handheld sporting implementthat is far from where an athlete holds the shaft. The knob of thehandheld sporting implement, as the term is used herein, refers to theproximal end of the shaft of the handheld sporting implement that isclose to where an athlete grips or holds the shaft. Depending upon thespecific handheld sporting implement, the knob may or may not be widerthan the portion of the shaft that is held by an athlete. For example,while the knob of a baseball bat is typically wider than the adjacentportion of the bat that is gripped by an athlete, that is not always thecase with a cricket bat or a golf club.

In the description that follows, like numerals or reference designatorswill be used to refer to like parts, steps or elements throughout. Inaddition, the first digit of a three digit reference number, or thefirst two digits of a four digit reference number, identifies thedrawing in which the reference number first appears. Further, it isnoted that the terms “identifying” and “determining” are often usedinterchangeably herein.

FIG. 1A-1C will initially be used to describe equipment that can be usedto implement the embodiments described herein, specifically where thehandheld sporting implement is a baseball bat. Nevertheless, it shouldbe appreciated that embodiments of the present technology canalternatively be used to track a path and/or orientation of other typesof handheld sporting implements that are swung by an athlete, including,but not limited to, a cricket bat, a golf club, or a hockey stick.

FIG. 1A depicts a baseball park 100 and equipment for obtaining videoimages, which can also be referred to herein as frames of video, videoframes, or simply as frames or images. The baseball park 100 can be abaseball stadium or a smaller facility, and includes a playing field102. The playing field 102 can be arranged according to standard rulesof baseball, e.g., as described in the “Official Baseball Rules” ofMajor League Baseball (MLB). A baseball game can be a game which isplayed according to these rules or similar rules. The fair territoryboundaries of the playing field are the foul lines 114 and 124, theoutfield boundary 126 which may be a fence or wall, and the semicircle107 around home plate 104. Line 130 is the boundary between the outfield138 and the infield. The infield includes a square/diamond region(including two sides 120 and 116) between the four bases 104, 112, 118and 122. The infield also includes a curved region 136 which is betweenthe regions 134 and 138. Also provided are: a left-side batter's box106, a right-side batter's box 108, a catcher's box 110, a first basecoach's box 140, a third base coach's box 142, a pitcher's mound 132,on-deck circles 144 and 146, and dugouts 150 and 148.

A number of video cameras obtain video images of the game as ittranspires in the baseball park 100. The video cameras can be, e.g.,cameras dedicated for use in tracking, or television video cameras thatare also used to televise and/or record a game, or a combinationthereof. Any one of the cameras can have a fixed location or can bemovable, and any one of the cameras can have a fixed or variablepan-tilt-zoom (PTZ). For example, in FIG. 1A three cameras 160A, 160Band 160C are depicted outside the fair territory of the playing field102 (and thus, in foul territory), with the camera 160A generally facingthe base path between home plate 104 and first base 112, the camera 160Bbehind and generally facing home plate 104, and the camera 160Cgenerally facing the base path between home plate 104 and third base122. The video images captured by each of the cameras 160A, 160B and160C preferably include the full stance or pose of the athlete (e.g., abaseball player) and the cameras collectively preferably capture thefull range of swinging motion, but that need not be the case. Where eachof the cameras 160A, 160B and 160C is located at a different positionthan one another, the images captured by the different cameras willdiffer from one another, despite including common objects within theirimages. The video images captured by of each of the cameras 160A, 160Band 160C may also include the pitcher's mound 132, so that the camerasare capable of being used to capture video images of a baseball as ittravels from the pitcher's mound 132 to home plate 104. More generally,if the object being swung at is moving, the video images captured byeach of the cameras 160A, 160B and 160C may preferably include theobject (e.g., ball or puck) to be struck by the handheld sportingimplement (e.g., baseball bat or hockey stick) as the object travelstoward the swinging handheld sporting implement, so that the sameimages, if desired, can also be used to track the object at which thehandheld sporting implement is being swung. The cameras 160A, 160B and160C can be referred to collectively as cameras 160, or individual as acamera 160. In certain embodiments, one or more of the cameras 160 maybe located at a different height than one or more of the other camera(s)160. One or more of the cameras 160 may have different lenses, zoom,etc., than the other cameras. Further, various different types ofcameras 160 may be used in various different combinations. While threecameras 160 are depicted, more or fewer than three cameras 160 canalternatively be used, so long as there are at least two cameras 160. Inone approach, two to six cameras 160, capturing color or monochromeimages, can be used. A processing facility 164 receives and processesframes of video images from the cameras 160. In one approach, theprocessing facility 164 is a mobile facility such as a truck which isparked outside the baseball park. The processing facility 164 cansubsequently transmit the captured images and other information via anantenna 145, to another location such as a television broadcastfacility. In another approach, the processing facility can be remotefrom the baseball park 100. Or, the processing facility can be apermanent facility, neither mobile nor remote, such as one which isinside the baseball park. The cameras 160 can provide captured images orframes to the processing facility via wired or wireless communicationlinks, or a combination thereof, which may or may not include theInternet.

In accordance with certain embodiments, the cameras 160 are allsynchronized so that each of the cameras 160 obtains video images of anathlete swinging a baseball bat, with at least two of the cameras 160being at different positions, at common points in time (i.e., at commoninstants that the handheld sporting implement is being swung). This waytriangulation and/or other techniques can be used to determine thelocation of the head, knob and/or shaft of the baseball bat inthree-dimensional (3D) space from the two-dimensional (2D) images of thebaseball bat captured at the same times by the different cameras 160, aswill be appreciated from the description below. In alternativeembodiments, the various cameras 160 may be unsynchronized relative toone another.

Additional cameras 160 can be used as well to increase the accuracyand/or robustness of the tracking. The cameras can be, e.g., specialpurpose machine vision cameras. Alternatively, or additionally,television broadcast cameras can be used. Such broadcast camerastypically capture thirty frames or sixty fields per second, but maycapture frames and/or fields at other rates as well (for exampleprogressive cameras typically capture sixty frames per second, and superslow motion cameras capture much higher frame rates). Other cameras thatcapture multiple frames of video images per second can alternatively beused. The locations of objects in the baseball park, includingparticipants, a baseball bat, and a baseball, can be described in termsof a world coordinate system, also known as a free space coordinatesystem, which is fixed relative to the earth or other environment ofinterest, in one approach. The world coordinate system includesorthogonal directions represented by a Yw axis, an Xw axis, and a Zwaxis (not shown) which extends out of the page in FIG. 1A. An origin ofthe world coordinate system is chosen to be at the tip of home plate104, as an example. World coordinate space is an exemplary type of 3Dspace.

Each camera 160 can be provided with sensors which detect intrinsic andextrinsic parameters of the camera when these parameters are variable.Intrinsic parameters, such as focal length, lens distortion and zoomsetting represent characteristics of the camera design and settings, anddo not depend on the position and orientation of the camera in space.Extrinsic parameters, such as tilt or pan, depend on the position andorientation of the camera in space. Such sensors can be provided usingtechniques known to those skilled in the art. For example, pan and tiltsensors can be attached to a tripod head on which the camera is mounted.See, e.g., U.S. Pat. No. 5,912,700, issued Jun. 15, 1999, andincorporated herein by reference. The sensors can be used to determinewhere the camera is pointing and what it can see. The sensors can beused to determine where the camera is pointing and what it can see. Or,the cameras can be stationary and fixed so that they do not pan, tilt orzoom dynamically, in which case mathematical methods can be used todetect the extrinsic and intrinsic camera parameters. In certainembodiments, broadcast cameras with a pan-tilt-zoom (PTZ) capabilitycould be used for all of the tracking, part of the tracking, or inconjunction with stationary and fixed cameras to assist with thetracking.

It is possible to determine camera extrinsic and intrinsic parameterswithout sensors, e.g., as described in Tsai's method. See, e.g., Tsai,Roger Y. (1986) “An Efficient and Accurate Camera Calibration Techniquefor 3D Machine Vision,” Proc. of IEEE Conf. on Computer Vision andPattern Recognition, Miami Beach, Fla., 1986, pp. 364-374. For example,one approach to determine the intrinsic and extrinsic parameters of acamera involves placing physical markers, known as fiducials, in variousmeasured or known locations in the event facility such that thefiducials can be seen clearly enough to be identified from the cameraimages, and at least one fiducial will always be visible to the camerawhile the camera is pointed at the event facility. A computer usingoptical recognition technology can find the fiducial in the video frameand, based on the mark's size, shape, color and/or position in the videoframe, determine the camera parameters. Another approach to determiningintrinsic and extrinsic parameters of a camera involves placingfiducials in various measured or known locations in the event facilitysuch that each fiducial looks different, but the fiducials may beremoved after camera parameters have been determined. A computerimplementing a camera parameter estimation algorithm based on manualuser interaction rather than, or in addition to, image recognition candetermine camera parameters.

FIG. 1B depicts further details of the processing facility 164 andcameras 160 of FIG. 1A. The computer system 170 is a simplifiedrepresentation of a system which might be used at the processingfacility 164 (FIG. 1A), for example. The computer system 170 includes astorage device 171 such as a hard disk or portable media, a networkinterface 172 for communicating with other computer systems, one or moreprocessors 173 for executing software instructions, a working memory 174such as RAM for storing the software instructions after they are loadedfrom the storage device 171, for example, camera interfaces 175A, 175Band 175C, and a user interface display 176. The camera interfaces 175A,175B and 175C can be referred to collectively as camera interfaces 175,or individually as a camera interface 175. The storage device 171 may beconsidered to be a processor readable storage device having processorreadable code embodied thereon for programming the processor 173 toperform methods for providing the functionality discussed herein. Theuser interface display 176 can provide information to a human operatorbased on the data received from the cameras 160 via the camerainterfaces 175. The user interface display 176 can use any known displayscheme, whether graphical, tabular or the like. In addition to anon-screen display, an output such as a hard copy from a printer can beprovided to report results. Results can also be reported by storing dataat the storage device 171 or other memory, e.g., for later use. Resultscould also be sent via the network interface 172 and the Internet orother wide area network, to another, central storage location. Incertain embodiments, the results can include a digital record of abaseball game or portions thereof.

An example camera 160A includes intrinsic parameter sensors 179 andextrinsic parameter sensors 180. The intrinsic parameter sensors 179 canidentify a zoom setting, whether an extender is used and so forth. Theextrinsic parameter sensors 180 can identify an orientation of thecamera 160A, such as a pan and tilt of the camera. Note that sensors arenot needed when the parameter of concern is not changing. The camera160A communicates image data, whether analog or digital, in addition todata from the intrinsic parameter sensors 179 and the extrinsicparameter sensors 180 to the computer system 170 via the camerainterface 175. The image data can include video images captured by thecamera 160A. Similarly, the other cameras 160B and 160C, which can eachinclude intrinsic parameter sensors and extrinsic parameter sensors, cancommunicate image data to the camera interfaces 175B and 175C. Data frommore or fewer than three cameras 160 can be received as well.

Further, the functionality described herein may be implemented using oneor more processor readable storage devices (e.g., 171 and 174) havingprocessor readable code embodied thereon for programming one or moreprocessors to perform the processes described herein. The processorreadable storage devices can include non-transitory, tangible computerreadable media such as volatile and nonvolatile media, removable andnon-removable media. Computer readable media includes volatile andnonvolatile, removable and non-removable media implemented in any methodor technology for storage of information such as computer readableinstructions, data structures, program modules or other data. Computerreadable media includes RAM, ROM, EEPROM, flash memory or other memorytechnology, CD-ROM, digital versatile disks (DVD) or other optical diskstorage, magnetic cassettes, magnetic tape, magnetic disk storage orother magnetic storage devices, or any other non-transitory, tangiblemedium which can be used to store the desired information and which canbe accessed by a computer.

FIG. 1C depicts a relationship between camera, image and worldcoordinate systems, for use by the processing facility of FIG. 1A fortracking a path of a handheld sporting implement that is swung by anathlete. A camera coordinate system 196, which includes orthogonal axesX.sub.C, Y.sub.C and Z.sub.C in three dimensions, is fixed relative tothe camera. The origin of the coordinate system may be at the center ofprojection of the lens, in one possible approach, modeling the camera asa pinhole camera. An image coordinate system 190, also referred to aspixel space or image space, includes orthogonal axes X and Y intwo-dimensions, and is fixed relative to a captured image 192. A worldcoordinate system 197, also referred to as world space, which includesorthogonal axes X.sub.W, Y.sub.W and Z.sub.W, is fixed relative to,e.g., the earth, a baseball park or other event site, or other referencepoint or location. Image space is an example of a 2D space, whereasworld space is an example of a 3D space. Generally, it is desirable todescribe the position and/or path of the tracked object 191 (e.g., atracked baseball) in the world coordinate system 197, which is a 3Dcoordinate system, as this is typically the coordinate system in whichits motion is most relevant to the user, and allows easier integrationof the information from several cameras. The line of position 194 is animaginary line which extends from the origin of the camera coordinatesystem 196, which as noted above can be the center of projection of thelens, through a pixel in the image 192, intersecting the pixel at apoint 193, and through the tracked object 191. Each pixel in the image192 corresponds to a different line of position (LOP). Point 193 in thecaptured image 192 represents the location of an object 191 (e.g., ahead of a baseball bat) in the image. The location of the object in theimage 192 can be represented by coordinates (sx, sy) in a coordinatesystem which has its origin at a corner of the image, in one approach.The coordinates may identify the center of the object. When the objectis a tracked human participant, characteristics such as the outline ofthe participant can be detected.

Further, the line of position can be represented by a 3-D vector (LOP)which has unity magnitude, in one approach. The vector can be defined bytwo points along the LOP. Alternatively, the vector can be defined byone point along the LOP, if the center of projection of the lens isknown. The vector can be represented in the world coordinate system 197using an appropriate transformation from the image coordinate system.The Z.sub.C axis of the camera coordinate system, which is the opticalaxis 195 of the camera, intersects the captured image at a pointrepresented by coordinates (0.sub.x, 0.sub.y). A two-dimensionalcoordinate system extending from (0.sub.x, 0.sub.y) can also be defined.

The camera registration process involves obtaining one or moretransformation matrices which provide a conversion between the imagecoordinate system 190 and the world coordinate system 197. Furtherinformation can be found in E. Trucco and A. Verri, “Introductorytechniques for 3-D computer vision,” chapter 6, Prentice Hall, 1998,U.S. Pat. No. 5,912,700, issued Jun. 15, 1999, and U.S. Pat. No.6,133,946, issued Oct. 17, 2000, each of which is incorporated herein byreference.

In accordance with certain embodiments of the present technology, two ormore cameras 160 are used to capture video images of an athlete applyinga swinging motion to a bat, and more generally, to a handheld sportingimplement. Using computer vision techniques, the far end of the handheldsporting implement, otherwise referred to as the head, is identified inmany video images (also referred to as video frames) containing themoving handheld sporting implement and the moving athlete holding thehandheld sporting implement. Using multiple views of this movement, athree-dimensional position of the head can be identified in many, butperhaps not all, instants corresponding to the time each video image wascaptured. Using these measurements of 3D positions, a smoothly-varyingrepresentation of the motion of the swinging implement is computed overthe course of the movement. Such embodiments will initially be describedwith reference to the high level flow diagram of FIG. 2. Morespecifically, FIG. 2 will be used to describe certain methods fortracking a path of a handheld sporting implement that is swung by anathlete, wherein the handheld sporting implement has a shaft extendingbetween a head and a knob.

Referring to FIG. 2, step 202 involves receiving two or more differentsets of video images of a handheld sporting implement being swung by anathlete, wherein each of the different sets of video images is capturedusing a different camera, and wherein at least two of the differentcameras have a different position. For example, referring back to FIG.1A, step 202 can include receiving a first set of video images of anathlete swinging a bat captured using the camera 160A, receiving asecond set of video images of the athlete swinging the bat capturedusing the camera 160B, and receiving a third sets of video images of theathlete swinging the bat captured using the camera 160C. In thisexample, the location of the camera 160A is in foul territory betweenhome plate and first base, the location of the camera 160B is in foulterritory behind home plate, and the location of the camera 160C is foulterritory between home plate and third base. For the purpose of thisdescription, it can be assumed that step 202 involves receiving threesets of video images of an athlete swinging a bat, from the cameras160A, 160B and 160C, wherein each set of images includes thirty videoimages. As noted above, the cameras 160A, 160B and 160C can collectivelybe referred to as the cameras 160, or individually as a camera 160.

Referring again to FIG. 2, step 204 involves identifying one or moremotion regions within each of a plurality of the video images in each ofthe different sets of video images. For the purpose of this description,it will be assumed that step 204 involves identifying one or more motionregions within each of the thirty video images in each of the three setsof video images, and thus, involves identifying motion region(s) withineach of the ninety video images. FIG. 3A illustrates an exemplary videoimage (also known as a video frame) obtained by the camera 160A in FIG.1A. FIG. 3B illustrates motion regions identified by comparing the videoimage in FIG. 3A to a preceding video image and/or a following videoimage in a sequence of the video images captured by the camera 160A. Ascan be appreciated from FIGS. 3A and 3B, the motion regions include thebat (and more generally, the handheld sporting implement) that is beingswung, and the athlete's arms that are swinging the bat, and portions ofthe athletes legs that move when the athlete swings the bat. FIG. 3C,shown to the right of FIG. 3B, will be discussed below when describingstep 206 in FIGS. 2 and 5. Additional details of step 204, according toa specific embodiment of the present technology, are described belowwith reference to FIG. 4.

Referring again to FIG. 2, step 206 involves, for at least a subset(i.e., all or some) of the video images included in each of thedifferent sets of video images, identifying one or more candidatelocations in 2D space of the head of the handheld sporting implementwithin the identified motion region(s) of the video image. Referring toFIG. 3C, the points labeled 312 a, 312 b, 312 c, 312 d, 312 e and 312 fare exemplary candidate locations in 2D space of the head of the batwithin the identified motion region(s) shown in FIG. 3B of the videoimage shown in FIG. 3A. Additional details of step 206, according to aspecific embodiment of the present technology, are described below withreference to FIG. 5. Each of the candidate locations in 2D space of thehead of the handheld sporting implement can be specified by the pixelcoordinates (e.g., sx, sy) for each image in a set of images.

Referring again to FIG. 2, step 208 involves identifying (from thecandidate locations in 2D space of the head of the handheld sportingimplement) a probable location in three-dimensional (3D) space of anidentifiable portion (e.g., the head) of the handheld sportingimplement, for each of a plurality of instants during which the handheldsporting implement was swung by the athlete. The plurality of instantscan be all of the times that video images were captured using two ormore of the cameras 160A, 160B and 160C, or just some of those times.Further, it is noted that for some of the instants, the bat head (orother identifiable portion of the handheld sporting implement) may becaptured by fewer than all of the cameras 160A, 160B and 160C, e.g., dueto the player or something else obstructing the view of the bat headfrom certain cameras during certain instants, or for other reasons.Additional details of step 208, according to a specific embodiment ofthe present technology, are described below with reference to FIG. 10.For much of the following description, it is assumed that theidentifiable portion of the handheld sporting implement is its head,however that need not be the case. For another example, a ring that ispainted around a portion (e.g., the center) of a bat or other handheldsporting implement can be the identifiable portion.

Still referring to FIG. 2, step 210 involves approximating a piecewise3D trajectory of at least the head (and/or any other identifiableportion) of the handheld sporting implement based on the probablelocations in 3D space of the head (and/or any other identifiableportion) of the handheld sporting implement (identified at step 208 foreach of the plurality of instants during which the handheld sportingimplement was swung by the athlete). Additional details of step 210,according to a specific embodiment of the present technology, aredescribed below with reference to FIG. 12. In certain embodiments,extrapolation techniques can be used to extrapolate the path and/ororientation of the handheld sporting implement beyond (prior to and/orafter) the timespan during which images of the swung handheld sportingimplement are captured using the cameras 160.

Additional details of step 204, according to a specific embodiment ofthe present technology, are now described with reference to FIG. 4. Morespecifically, FIG. 4 is used to describe additional details of how toidentify one or more motion regions within a video image. Referring toFIG. 4, in the center at the top of the page is shown an exemplary videoimage for which one or more motion regions are to be identified. Thisvideo image shown in the center at the top of FIG. 4, which can bereferred to as the present or current image, is the same as the videoimage shown in FIG. 3A. To the left and right of the present video imageare blocks representative of, respectively, previous and following videoimages within a sequence of video images captured using the same camera160.

Still referring to FIG. 4, at step 402 the previous, present andfollowing video images are each low-pass filtered, which results in ablurring of each of the images. The purpose of step 402 is to reduceimage noise. Step 402 can be performed, e.g., by applying a Gaussianblur to each of the previous, present and following video images, but isnot limited thereto. At step 404, image subtractions are performed todetermine the difference between the present video image and theprevious video image, and to determine the difference between thepresent video image and the following video image. At step 406, apixel-wise root-squared operation is performed on the results of theimage subtractions performed at steps 404 to thereby diminish smallervalues, amplifier larger values and invert negative values. Step 406could be replaced with a pixel-wise absolute-difference operation, whichwould likely be faster and produce very similar results. At step 408,the results from step 406 are normalized by stretching pixel values to afull grayscale range. At step 410 a binary threshold is applied to theresults of step 408 to convert pixels to either white or black. Such abinary threshold can be the middle of the full grayscale range, butother binary thresholds are also possible. In an embodiment of step 410,pixels having a grayscale above the binary threshold are converted towhite, with all other pixels being converted to black. At step 412, alogical “AND” operation is applied to the results of step 410 to therebymaintain only pixels that are white in the results of both instances ofstep 410. At step 414, the result of the logical “AND” operationperformed at step 412 is masked with the original (i.e., present) image(shown in the center at the top of the page, and to the right of thestep labeled 412) in order to maintain original pixels at locations ofwhite mask pixels. At step 416, a further binary threshold is applied tothe results of the masking at step 414, to thereby cause pixels in whichboth motion was detected and which were brightly colored in the originalscene (such as those of the brightly-colored baseball bat) to berepresented in white, with all other pixels represented in black. Thethreshold used at step 416 can be inverted to detect darkly-colored bats(or other handheld sporting implements), where all pixels at locationsof the motion mask in the original scene that have pixel intensitiesbelow the threshold level are converted to white, and all others areconverted to black. The result of step 416 is shown at the bottom of thepage, which is the same as that which is shown in FIG. 3B. The stepsdescribed with reference to FIG. 4 can be performed for each (or some)of the video images included in each (or some) of the different sets ofvideo images captured by the different cameras 160A, 160B and 160C, tothereby identify one or more motion regions in each of the video images,and more generally, to perform step 204 in FIG. 2.

Additional details of step 206, according to a specific embodiment ofthe present technology, are now described with reference to FIG. 5. Morespecifically, FIG. 5 is used to describe additional details of how toidentify one or more candidate locations (in 2D space) of a head (and/orany other identifiable portion) of a handheld sporting implement withinidentified motion region(s) of a video image. Referring to FIG. 5, step502 involves identifying one or more candidate shafts of the handheldsporting implement (e.g., bat) within the identified motion region(s) ofthe video images. Step 502 can be performed by outlining the regions ofmotion within each of a plurality of the video images in each of thesets of video images, and then identifying nominally straight lineswithin the outlined regions of motion. Exemplary outlined regions ofmotion within a video image are labeled 304 in FIG. 3C. Exemplarynominally straight lines (that are identified within the outlinedregions of motion 304) are labeled 306 a, 306 b and 306 c in FIG. 3C. Inaccordance with an embodiment, a Canny edge detector algorithm is usedto perform the outlining of the regions of motion. In accordance with anembodiment, a Hough transform is used to identify the nominally straightline segments within the outlined regions of motion. The use ofalternative and/or additional algorithms are also possible. In order toavoid duplicate lines, nominally parallel lines within close proximitycan be merged, e.g., by averaging the lines, such as the two long,nominally parallel lines labeled 306 a.

Referring again to FIG. 5, step 504 involves identifying an approximatecenter of elliptical arc motion of the handheld sporting implement thatis swung by an athlete. Additional details of step 504, according to aspecific embodiment of the present technology, are described below withreference to FIGS. 6 and 7. Still referring to FIG. 5, step 506 involvesusing the approximate center of elliptical arc motion (identified atstep 504) and one or more candidate shafts (identified at step 502) toidentify the one or more candidate locations (in 2D space) of the headof the handheld sporting implement within the identified motionregion(s) of the video images. Additional details of step 506, accordingto a specific embodiment of the present technology, are described belowwith reference to FIG. 8.

Additional details of step 504, according to a specific embodiment ofthe present technology, are now described with reference to FIGS. 6 and7. In FIG. 6, the dots shown therein represent candidate heads (oranother identifiable portion) of the handheld sporting implementdetermined from a plurality of video images captured using a single oneof the cameras 160A, 160B and 160C while an athlete swung the implement.Referring to the high level flow diagram of FIG. 7, at step 702 aspecified number (e.g., 5) of the smallest x-positions are identifiedand averaged. The dots within the dashed oval 602 in FIG. 6 correspondto the 5 smallest x-positions, and the dashed line 603 is representativeof the average of these smallest x-positions. At step 704, a specifiednumber (e.g., 5) of the largest x-positions are identified and averaged.The dots within the dashed oval 604 in FIG. 6 correspond to the 5largest x-positions, and the dashed line 605 is representative of theaverage of these largest x-positions. At step 706, the average valuesdetermined at steps 702 and 704 are averaged to determine an average ofthe largest and smallest x-positions, which is represented by the dashedline 607. At step 708, a specified number (e.g., 5) of the smallesty-positions are identified and averaged. The dots within the dashedregion 608 in FIG. 6 correspond to the 5 smallest y-positions, and thedashed line 609 is representative of the average of these smallesty-positions. At step 710, a specified number (e.g., 5) of the largesty-positions are identified and averaged. The dots within the dashedregion 610 in FIG. 6 correspond to the 5 largest y-positions, and thedashed line 611 is representative of the average of these largesty-positions. At step 712, the average values determined at steps 708 and710 are averaged to determine an average of the smallest and largesty-positions, which is represented by the dashed line 613. At step 714,an approximate center of elliptical arc motion of the handheld sportingimplement, which center is represented by the triangle 615, isdetermined to be the position corresponding to the average of thelargest and smallest x-positions (as determined at step 706) and theaverage of the largest and smallest y-positions (as determined at step712). The specific number of x- and y-positions that are averaged in thesteps in FIG. 7 can be more than or fewer than 5. Alternative techniquesfor determining a center of elliptical arc motion of a handheld sportingimplement that is swung by an athlete are possible and can be used withembodiments described herein.

Additional details of step 506, according to a specific embodiment ofthe present technology, are now described with reference to FIG. 8. Morespecifically, the high level flow diagram of FIG. 8 is used to explainhow an approximate center of elliptical arc motion, which was identifiedat step 504, can be used to identify one or more candidate locations (in2D space) of a head of a handheld sporting implement within identifiedmotion region(s) of a video image. Referring to FIG. 8, step 802involves identifying first and second ends of each of the one or morecandidate shafts. Referring briefly back to FIG. 3C, lines 306 a, 306 band 306 c are illustrative of exemplary candidate shafts. In FIG. 3C,the labels 312 a and 312 b point to the first and second ends of thecandidate shaft represented by the line 306 a, the labels 312 c and 312d point to the first and second ends of the candidate shaft representedby the line 306 b, and the labels 312 e and 312 f point to the first andsecond ends of the candidate shaft represented by the line 306 c.Referring again to FIG. 8, step 806 involves, for each of the candidateshafts, identifying and filtering out the one of the first and secondends of the candidate shaft that is closer to the approximate center ofelliptical arc motion of the shaft, whereby the non-filtered out ones ofthe first and second ends remain as candidate locations in 2D space ofthe head of the handheld sporting implement within the identified motionregion(s) of the a video image. In FIG. 3C, the triangle labeled 315 isrepresentative of the approximate center of elliptical arc motion of theshaft as determined at step 504. Still referring to FIG. 3C, for thecandidate shaft 306 a, the end 312 b is filtered out since it is closerthan the other end 312 a to the triangle 315. For the candidate shaft306 b, the end 312 d is filtered out since it is closer than the end 312c to the triangle 315. For the candidate shaft 306 c, the end 312 e isfiltered out since it is closer than the end 312 f to the triangle 315.Accordingly, the remaining candidate locations (in 2D space) of the headof the handheld sporting implement are the ends 312 a, 312 c and 312 f.

Referring again to FIG. 8, step 806 involves applying one or moreheuristics to identify and filter out one or more of the candidatelocations of the head or another identifiable portion of the handheldsporting implement (remaining after step 804) that is/are determined tobe false positives. Such heuristics can use the approximate center ofelliptical arc motion that was identified at step 504. One heuristic caninvolve enforcing a unidirectional, elliptical path of motion in thetime domain, as it is expected that a head of a handheld sportingimplement will move in a single direction and in an approximatelyelliptical path when being swung. Another heuristic can involveenforcing minimum and/or maximum object length thresholds. For example,wherein the identifiable portion of the handheld sporting implement isthe head, for each of the candidate heads (e.g., bat heads), theEuclidean distance from the approximate center of elliptical arc motionto the candidate bat head can be calculated. The calculated distancescan then be compared to a minimum distance threshold which specifies aminimum expected distance that a bat head will be from the approximatecenter of elliptical arc motion when a bat is swung. The calculateddistances can also be compared to a maximum distance threshold whichspecifies a maximum expected distance that the bat head will be from thecenter of elliptical arc motion when the bat is swung. Candidate headsthat have a distance (from the approximate center of elliptical arcmotion) that is less than the minimum distance threshold or greater thanthe maximum distance threshold are filtered out. The use of additionaland/or alternative heuristics are also possible and within the scope ofembodiments of the present technology. The heuristics may depend uponwhat identifiable portion of the handheld sporting implement is beingidentified and tracked.

Referring now to FIG. 9, illustrated therein is a video image of anathlete holding a bat during the start of a swinging motion. The whitecircles that are superimposed on the video image shown in FIG. 9 areillustrative of candidate locations in 2D space of the head of the batthat were identified at step 206 for a plurality of video images thatfollowed the video image shown in FIG. 9, after false positives werefiltered out. The elliptical arc superimposed on the video image shownin FIG. 9 is illustrative of the trajectory of the head of the batduring the swing.

Additional details of step 208, according to a specific embodiment ofthe present technology, will now be described with reference to FIG. 10.More specifically, FIG. 10 is a high level flow diagram that is used todescribe how to identify from the candidate locations (in 2D space) ofthe head of the handheld sporting implement, a probable location in 3Dspace of the head of the handheld sporting implement for each of aplurality of instants during which handheld sporting implement was swungby an athlete. The steps described with reference to FIG. 10 areperformed for each of a plurality of instants during which the handheldsporting implement was swung by the athlete, so that the path of theswing can be approximated at step 210. As mentioned above, the pluralityof instants can be all of the times that video images of a swing werecaptured using two or more of the cameras 160A, 160B and 160C, or justsome of those times.

Referring to FIG. 10, step 1002 involves identifying different possiblecombinations of the remaining candidate locations (CL) in 2D space ofthe head (or other identifiable portion) of the handheld sportingimplement that are based on images captured using at least two differentones of the cameras, wherein no single combination should include two ormore candidate locations captured using the same camera. For example,assume that each of the cameras 160A, 160B and 160C captured a separatevideo image of an athlete swinging a bat at a same point in time (i.e.,at a same instant), due to the cameras being synchronized, wherein thecaptured video images can be respectively referred to as video images A,B and C. Also assume that after filtering out candidates that were falsepositives (e.g., at steps 704 and 706), the video image A included twocandidate locations for the head (referred to as CL-A-1 and CL-A-2), thevideo image B included one candidate location for the head (referred toas CL-B), and the video image C included two candidate locations for thehead (referred to as CL-C-1 and CL-C-2). The different possiblecombinations of these candidate heads from the video images A, B and C,captured using at least two different ones of the three cameras (with nosingle combination including two or more candidates captured using thesame camera), include the following combinations:

-   -   CL-A-1 and CL-B;    -   CL-A-2 and CL-B;    -   CL-A-1 and CL-C-1;    -   CL-A-2 and CL-C-1;    -   CL-A-1, CL-B and CL-C-1;    -   CL-A-1, CL-B and CL-C-2;    -   CL-A-2, CL-B and CL-C-1;    -   CL-A-2, CL-B and CL-C-2;    -   CL-B and CL-C-1; and    -   CL-B and CL-C-2.

If for a same point in time (i.e., instant) there is only one candidatelocation of the head per image captured by N separate synchronizedcameras (e.g., 160), then the following equation can be used tocalculate the amount of all possible combinations of candidate locationsfor the head, where each combination includes either zero or onecandidate location for the head per separate camera, and where eachcombination includes candidate locations for the head associated with atleast two separate cameras:

${\# \mspace{14mu} {of}\mspace{14mu} {combinations}} = {\sum\limits_{n = 2}^{N}{\frac{N!}{{n!}{( {N - n} )!}}.}}$

Step 1004 involves, for each of the different possible combinations (ofremaining candidate locations in 2D space of the head of the handheldsporting implement) identified at step 1002, determining a correspondingline of position (LOP) in 3D space for each candidate location in 2Dspec of the head of the handheld sporting implement (that is included inthe possible combination). In accordance with an embodiment, atransformation matrix is used to perform step 1004, wherein thetransformation matrix is determined based on knowledge of parameters ofthe cameras 160A, 160B and 160C, which parameters can include position,orientation, angular width, and lens distortion characteristics of eachof the cameras, but are not limited thereto. Exemplary orientation typeparameters can include tilt and/or pan of each of the cameras. In otherwords, at step 1004, corresponding candidate head locations in 2D space,as determined from 2D images captured using different cameras, can eachbe transformed into an LOP in 3D space using a transformation matrix. Anexemplary transformation matrix M is shown below, wherein thetransformation matrix M relates a 2D image coordinate system to a 3Dworld coordinate system:

$M = {\begin{pmatrix}{m\; 00} & {m\; 01} & {m\; 02} & {m\; 03} \\{m\; 10} & {m\; 11} & {m\; 12} & {m\; 13} \\{m\; 20} & {m\; 21} & {m\; 22} & 1\end{pmatrix}.}$

The values in the transformation matrix M, for use with one of thecameras 160, can be determined during a calibration procedure for thatcamera 160, which can also be referred to as registration. Thecalibration procedure can involve aiming a camera 160 at differentreference fiducials in an event facility (e.g., baseball park), whereinactual locations of the reference marks are known, e.g., using surveyingequipment. Values within the matrix can then be solved for using thecapturing images of the fiducials, parameters of the camera used tocapture the images of the fiducials, and the actual locations of thefiducial (e.g., as determined using surveying equipment). For oneexample, the tip of home plate may be a fiducial. In an embodiment whereunsynchronized cameras are implemented, the conversion from 2D space to3D space may involve fitting screen points visible in each camera to amodel of a swing. This may be done in ways similar to those described incommonly assigned U.S. Pat. No. 8,335,345, entitled “Tracking an Objectwith Multiple Asynchronous Cameras,” which is incorporated herein byreference, but other techniques can be used as well.

Step 1004 is performed such that there are at least two lines ofposition associated with each combination. This can be better understoodwith reference to FIG. 11, which shows lines of position 1102A, 1102Band 1102C from cameras 160A, 160B and 160C, respectively, wherein eachline of position 1102 represents an imaginary straight line that extendsfrom a camera 160 to a tracked object (the head of a handheld sportingimplement, in this example) at a given point in time, and identifies alocus of points at which the object could be located based on thecamera's observation. Thus, for camera 160A, lines of position extendfrom the camera 160A to the different positions of the tracked object(the head of a handheld sporting implement, in this example) at thedifferent times the images of the object are captured by camera 160A.The example line of position (LOP) 1102A represents a line which extendsfrom the camera 160A through the tracked object (the head of a handheldsporting implement, in this example) at a single point in time. Theexample LOP 1102B represents a line which extends from the camera 160Bthrough the head of the handheld sporting implement at the same point intime, and the example LOP 1102C represents a line which extends from thecamera 160C through the head of the handheld sporting implement at thesame point in time.

Referring again to FIG. 10, step 1006 involves determining, as theprobable location in 3D space of the head of the handheld sportingimplement for an instant, a location in 3D space where distances betweenthe location and the lines of position are at a minima. In FIG. 11, thecalculated probable location 1104 in 3D space of the head of thehandheld sporting implement is at distances d1, d2 and d3 from the linesof position 1102A, 1102B and 1102C, respectively. Geometric principlescan be used to obtain equations which express the distance (or square ofthe distance) from each of the lines of position to an unknown position.A minima for the equations can then be determined to obtain the probablelocation in 3D space of the head of the handheld sporting implementwhich meets the distance metric. For example, a distance metric canexpressed as (d1+d2+d3) or (d1.sup.2+d2.sup.2+d3.sup.2), and can beminimized to find the probable location in 3D space of the head of thehandheld sporting implement. There are multiple different ways in whichdistance metrics can be defined. Also, there are multiple different waysin which a minima can be defined. The probable location in 3D space canalso be determined by a metric that takes into account the number ofLOPs and quality of candidate heads identified in 2D space. In certainembodiments, this may enable outlier candidate heads and/or LOPs to beremoved from a pool of LOPs, and subsequent iteration of prior steps tobe performed, if necessary or desired. The algorithm described withreference to FIG. 10 is an example of a clustering algorithm. Othertypes of clustering algorithms can alternatively be used. As notedabove, the portion of the camera from which the LOP extends may be thecenter of projection of the lens of the camera.

Additional details of step 210, according to a specific embodiment ofthe present technology, are now described below with reference to FIG.12. As explained above in the discussion of FIG. 2, step 210 involvesapproximating a piecewise 3D trajectory of at least the head of thehandheld sporting implement based on the probable locations in 3D spaceof the head of the handheld sporting implement (identified at step 208for each of the plurality of instants during which the handheld sportingimplement was swung by the athlete). In FIG. 12, the dots along thejagged line 1202 are probable locations in 3D space of the head of thehandheld sporting implement, which were identified at step 208 for eachof the plurality of instants during which the handheld sportingimplement was swung by the athlete. In FIG. 12, the smooth line 1204 isillustrative of an approximated piecewise 3D trajectory of the head ofthe handheld sporting implement that is based on the probable locationsin 3D space of the head of the handheld sporting implement. Inaccordance with an embodiment, a B-spline algorithm is used toapproximate the piecewise 3D trajectory of the head of the handheldsporting implement. Instead of using a B-spline algorithm, another typeof curve fitting algorithm can be used. In another embodiment, a physicsmodel of an athlete swinging a bat (or some other handheld sportingimplement) can be determined and used to approximate the piecewise 3Dtrajectory of the head of the handheld sporting implement. Othervariations are also possible, and within the scope of embodiments of thepresent technology. It is noted that in certain embodiments a piecewise3D trajectory can include multiple pieces that collectively cover theentire trajectory, while in other embodiments a piecewise 3D trajectorycan include a single piece that covers the entire trajectory. In otherwords, a sequence of functions can be used to approximate the piecewise3D trajectory in successive pieces, or a single function can cover theentire piecewise 3D trajectory.

The probable location in 3D space of the head of the handheld sportingimplement that is selected (at step 206) for each of a plurality ofinstants in time during which the handheld sporting implement is swungcan, if desired, be converted back into 2D space (e.g., using an inverseof the transformation matrix M), and superimposed on one or more videoimages of the handheld sporting implement being swung. Examples of thisare shown in FIGS. 13A and 13B, in which the open or non-filled incircles represent candidate locations in 2D space of the baseball bathead (as determined at step 208), and the solid or filled in circlesrepresent the probable locations in 3D space of the bat head (asdetermined at step 208). The 2D screen or image space shown in FIG. 13Ais captured by the camera 160A. The 2D screen or image space shown inFIG. 13B is captured by the camera 160B. More generally, a 2D video ofan athlete swinging a handheld sporting implement can be enhanced tovisually show the path of the entire shaft, or just the head, of thehandheld sporting implement. In certain embodiments, an error score canbe computed after back-projection into 2D space, which can involvecomputing a difference between an interpolated line passing roughlythrough the solid circles and the solid circles themselves. If aprobable location has an error exceeding a specified threshold, then itcan be discarded. Subsequent iteration of prior steps may be performedif necessary or desired. Other embodiments could discard a probablelocation by evaluating it in 3D space. One example would be a measure ofhow tightly the candidate LOPs are clustered; if that measurementexceeds a specified threshold, then that particular combination of LOPscan be discarded.

The above discussion described how to track the path of the head of ahandheld sporting implement (e.g., a baseball bat) that is swung by anathlete. Such embodiments can be extended to enable tracking of the pathof the entire handheld sporting implement (e.g., an entire baseball bat)that is swung. Such additional steps can be performed at the same timeas some of the steps described above, e.g., with reference to FIG. 2, orthey can be performed at a later time than some of the steps describedabove, depending upon implementation. More generally, a bat head searchcan be extended to a full bat search by means of locating screen-spacecorrespondences of a pair of points (corresponding to the head and theknob), and the line between them. For example, in FIG. 3C, linesdetected in a motion image are illustrated. Those lines are defined bytwo endpoints, which include the aforementioned bat head as well as theend nearest the hands, which is referred to herein as the knob, as notedabove.

Referring to FIG. 2, as part of step 206, in parallel with step 206, orat some other time, one or more candidate locations in 2D space of theknob of the handheld sporting implement can also be identified withinthe identified motion region(s) of the video image, for at least asubset of the video images included in each of the different sets ofvideo images. Still referring to FIG. 2, as part of step 208, inparallel with step 208, or at some other time, a probable location in 3Dspace of the knob of the handheld sporting implement can be identifiedfrom the candidate locations in 2D space of the knob of the handheldsporting implement, for each of the instants that a probable location in3D space of the head of the handheld sporting implement is/wasidentified. Still referring to FIG. 2, as part of step 210, in parallelwith step 210, or at some other time, a piecewise 3D trajectory of theknob of the handheld sporting implement can be approximated from theprobable locations in 3D space of the knob of the handheld sportingimplement identified for each of the plurality of instants during whichthe handheld sporting implement was swung by the athlete. Additionally,for each of the plurality of instants (for which a probable location ofthe knob is identified) a line is extended between the head and the knobto represent a probable location in 3D space of the shaft of thehandheld sporting implement for the instant. The length of the shaft canbe assumed to be the length between an identified head and an identifiedknob. Alternatively, a line representing the handheld sporting implement(e.g., a baseball bat) can be extrapolated, in 3D space, from theprobable location of the head through the probable location of the knob,terminating at a specified length (e.g., 33 inches) from the probablelocation of the head. It is practical to assume the bat length is 33inches, which is the length of a typical baseball bat, which typicallyonly varies +/−one inch. The one inch length variation at the knob endof the bat (or other handheld sporting implement) will beinconsequential, since this part of the bat only contacts a ball in rarecases.

By connecting interpolated, time-synchronized probable bat headlocations with probable bat knob locations, in the manner describedabove, 3D bat “lines” that pass axially through the center of the batare derived. From the perspective of each individual camera 160, lack ofdepth perception means an actual 3D bat line could lie anywhere in aplane extruded from the bat, both toward and away from the camera, asdepicted in FIG. 14 with a two-camera configuration. A line 1404 formedby the intersection of camera 160A's plane 1402A and camera 160B's plane1402B, is determined to be the 3D location of the bat.

In the above discussion, the embodiments described herein were typicallydescribed as being used to track the path and/or orientation of at leasta portion (e.g., the head) of a baseball bat that is swung by anathlete. However, as noted above, embodiments described herein canadditionally or alternatively be used to track the path and/ororientation of other types of handheld sporting implements that areswung by an athlete, such as, but not limited to, a cricket bat, a golfclub, or a hockey stick. Embodiments of the present technology can alsobe used to track the path and/or orientation of at least a portion of aracket type of handheld sporting implement, used, e.g., to play tennis,squash, or racquetball. When tracking a racket type of handheld sportingimplement, the entire racket head, the most distal end of the rackethead, or the throat of the tennis racket (which is typically v-shaped ortriangular), are examples of identifiable portions of a tennis racketthat can be tracked. It is also possible, and within the scope of theembodiments described herein, to use embodiments of the presenttechnology to track the path and/or orientation of portions of stillother types of handheld sporting implements that are swung by anathlete, such as, but not limited to, a cesta (used in Jai alai), a foil(used in fencing), a fighting stick (used in martial arts). Where ahandheld sporting implement does not include a relatively straightshaft, certain algorithms described above may be modified to account forthe different overall shape of the sporting implement. Also, where thehandheld sporting implement is intended to be maneuvered in specificmanners by an athlete, the expected trajectories can be taken intoaccount to modify some of the algorithms described above, to bettercustomize the algorithms for the sporting implement and sport.

Once the path and/or orientation of the head, shaft and/or otherportion(s) of a handheld sporting implement that is swung is/aredetermined, a graphical representation of the path and/or orientation ofthe head and/or shaft (and/or other portion(s)) can be superimposed overvideo images to enhance the video images. This can enable the analysisof an athlete's swing in order to improve and or otherwise adjust theathlete's swing to improve their performance and/or reduce the chance ofinjury.

Additionally, an athlete's swinging of a handheld sporting implement canbe correlated with results of the swing in order to detect correlationstherebetween and provide feedback to improve performance. For example,certain tendencies of an athlete can be identified in order to improvethe athlete's performance. Through such an analysis there can be adetermination, e.g., that an athlete tends to swing upward at certaintypes of pitches (e.g., fastballs), which results in pop-ups that areeasily caught by the defense. Based on this analysis, the athlete may beinstructed to try swinging in a more level manner when swinging atfastballs. For another example, there may be a determination that anathlete tends to swing too early in response to curveballs, and based onthat analysis the athlete may be instructed to try swinging a littlelater when swinging at curveballs. For still another example,embodiments described herein can be used to identify flaws in a golfer'sswing that result in the golfer slicing their drives, or the like. Theseare just a few examples, which are not intended to be all-encompassing.

Beneficially, embodiments described herein can track how an athleteswings a handheld sporting implement in an uninstrumented, unobstructedfashion, using as few as two cameras without requiring that an athletewear a special suit that is typically required by motion capturesystems. More specifically, many motion capture systems operate byhaving the actor (e.g., an athlete) wear special reflective orilluminating markers that make accurate motion capture possible. Insteadof using these markers—or in addition to using them—typical motioncapture systems often use a large collection of cameras (perhaps morethan ten) to record the action from a variety of different angles. Thosethat use such markers inevitably inhibit the motion of their actors (inthis case, athletes) via the placement of unfamiliar objects on theactors' bodies. These markers, along with the very controlled,constrained, and unfamiliar environments in which motion capture systemsexist, likely cause the actors to move in subtly different ways fromtheir natural, in situ behavior. Further, these markers can typicallynot be worn in an actual game or match, and thus, can typically only beused in very controlled practices.

Conventionally, swing analysis for sports, such as golf, wereconstrained to 2D space. For example, derivation of the golf club swingtrajectory is achieved in “Visual Golf Club Tracking for Enhanced SwingAnalysis,” N. Gehrig, British Machine Vision Conference, September 2003.However, this work involves strictly one camera, which must bepositioned directly in front of the athlete, where “in front” refers tothe camera being directed at the athlete's face when the athlete's headis not turned. This may not be practical in actual games or matches, andwould likely only be practical where the athlete is performing apractice swing that is not intended to make contact with a ball.Embodiments described herein enable swing analysis to be expanded into3D world space. Further, embodiments described herein can be used toperform swing analysis from video images captured during actual games ormatches, as well as during practice sessions.

Most of the functionality described herein may be implemented using oneor more processor readable storage devices having processor readablecode embodied thereon for programming one or more processors to performthe processes described herein. The processor readable storage devicescan include computer readable media such as volatile and nonvolatilemedia, removable and non-removable media. By way of example, and notlimitation, computer readable media may comprise computer storage mediaand communication media. Computer storage media includes volatile andnonvolatile, removable and non-removable media implemented in any methodor technology for storage of information such as computer readableinstructions, data structures, program modules or other data. Computerstorage media includes, but is not limited to, RAM, ROM, EEPROM, flashmemory or other memory technology, CD-ROM, digital versatile disks (DVD)or other optical disk storage, magnetic cassettes, magnetic tape,magnetic disk storage or other magnetic storage devices, or any othermedium which can be used to store the desired information and which canbe accessed by a computer. Communication media typically embodiescomputer readable instructions, data structures, program modules orother data in a modulated data signal such as a carrier wave or othertransport mechanism and includes any information delivery media. Theterm “modulated data signal” means a signal that has one or more of itscharacteristics set or changed in such a manner as to encode informationin the signal. By way of example, and not limitation, communicationmedia includes wired media such as a wired network or direct-wiredconnection, and wireless media such as acoustic, RF, infrared and otherwireless media. Combinations of any of the above are also includedwithin the scope of computer readable media.

Embodiments of the present technology have been described above with theaid of functional building blocks illustrating the performance ofspecified functions and relationships thereof. The boundaries of thesefunctional building blocks have often been defined herein for theconvenience of the description. Alternate boundaries can be defined solong as the specified functions and relationships thereof areappropriately performed. Any such alternate boundaries are thus withinthe scope and spirit of the claimed embodiments. For example, it wouldbe possible to combine or separate some of the steps shown in FIG. 2,and/or in other flow diagrams. Further, it may be possible to change theorder of some of the steps described above, without substantiallychanging the overall events and results, e.g., in instances where a stepdoesn't rely on the results of a previous step.

The foregoing detailed description of the invention has been presentedfor purposes of illustration and description. It is not intended to beexhaustive or to limit the invention to the precise form disclosed. Manymodifications and variations are possible in light of the aboveteaching. The described embodiments were chosen in order to best explainthe principles of the invention and its practical application, tothereby enable others skilled in the art to best utilize the inventionin various embodiments and with various modifications as are suited tothe particular use contemplated. It is intended that the scope of theinvention be defined by the claims appended hereto.

The above-mentioned examples are provided to serve the purpose ofclarifying the aspects of the invention, and it will be apparent to oneskilled in the art that they do not serve to limit the scope of theinvention. By way of example, the system can be applied to any objectapproaching a person. By nature, this invention is highly adjustable,customizable and adaptable. The above-mentioned examples are just someof the many configurations that the mentioned components can take on.All modifications and improvements have been deleted herein for the sakeof conciseness and readability but are properly within the scope of thepresent invention.

The invention claimed is:
 1. A method for tracking a sporting implementduring a sporting event, comprising: providing at least two camerasconstructed and configured for network communication with at least oneprocessor, wherein the at least two cameras are located at differentpositions in a sporting event facility; the at least two camerascapturing at least two different sets of images of the sportingimplement; the at least one processor receiving the at least twodifferent sets of images of the sporting implement from the at least twocameras; the at least one processor identifying at least one motionregion in the at least two different sets of images; the at least oneprocessor identifying a first location of at least one portion of thesporting implement within the at least one motion region; and the atleast one processor identifying a second location of the at least oneportion of the sporting implement based on the first location.
 2. Themethod of claim 1, wherein the first location comprises at least onecandidate location, and wherein the second location comprises at leastone probable location.
 3. The method of claim 2, further comprising theat least one processor approximating a trajectory of the sportingimplement based on a multiplicity of probable locations of the at leastone portion of the sporting implement identified for a multiplicity ofinstants.
 4. The method of claim 3, further comprising the at least oneprocessor superimposing a graphical representation of at least one partof the trajectory of the sporting implement over at least one of the atleast two different sets of images; and the at least one processordisplaying the graphical representation of the at least one part of thetrajectory of the sporting implement via a user interface display. 5.The method of claim 2, further comprising the at least one processorfiltering out at least one false positive candidate location based onheuristics.
 6. The method of claim 2, further comprising the at leastone processor identifying at least one combination of candidatelocations of the at least one portion of the sporting implement,determining a corresponding line of position in 3D space for eachcandidate location in each combination of candidate locations, anddetermining a location from which distances to at least two of amultiplicity of lines of position are at a minimum as a probablelocation in 3D space of the at least one portion of the sportingimplement.
 7. The method of claim 2, further comprising the at least oneprocessor identifying an approximate center of elliptical arc motion ofthe sporting implement; and the at least one processor identifying theat least one candidate location based on the approximate center ofelliptical arc motion of the sporting implement.
 8. The method of claim1, further comprising the at least one processor identifying at leastone motion region by comparing a sequence of images from the at leasttwo different sets of images of the sporting implement.
 9. The method ofclaim 1, wherein the at least one portion of the sporting implementcomprises a distal end of the sporting implement.
 10. The method ofclaim 1, wherein the at least two different sets of images of thesporting implement comprise a full range of motion of the sportingimplement.
 11. The method of claim 1, wherein the sporting implement isa handheld sporting implement, and wherein the at least two differentsets of images comprise a full stance of an athlete swinging thehandheld sporting implement.
 12. The method of claim 11, wherein the atleast two different sets of images comprise an object being swung atmoving toward the handheld sporting implement.
 13. A system for trackinga sporting implement during a sporting event, comprising: at least twocameras constructed and configured for network communication with atleast one processor; wherein the at least two cameras are located atdifferent positions in a sporting event facility; wherein the at leasttwo cameras are configured to capture at least two different sets ofimages of the sporting implement; and wherein the at least one processoris configured to receive the at least two different sets of images ofthe sporting implement from the at least two cameras, identify at leastone motion region in the at least two different sets of images, identifya first location of at least one portion of the sporting implementwithin the at least one motion region, and identify a second location ofthe at least one portion of the sporting implement based on the firstlocation.
 14. The system of claim 13, wherein the sporting implement isa handheld sporting implement, wherein the handheld sporting implementcomprises a distal end and a proximal end, and wherein the at least oneportion of the sporting implement comprises the distal end of thehandheld sporting implement.
 15. The system of claim 13, wherein the atleast one processor is further configured to approximate a trajectory ofthe sporting implement based on a multiplicity of second locations ofthe at least one portion of the sporting implement identified for amultiplicity of instants.
 16. The system of claim 15, wherein the atleast one processor is further configured to superimpose a graphicalrepresentation of the trajectory of the sporting implement over at leastone of the at least two different sets of images, and wherein the atleast one processor comprises a user interface display configured todisplay the graphical representation of the trajectory of the sportingimplement.
 17. A system for tracking a sporting implement during asporting event, comprising: at least two cameras constructed andconfigured for network communication with at least one processor;wherein the at least two cameras are located at different positions in asporting event facility; wherein the at least two cameras are configuredto capture at least two different sets of images of the sportingimplement; wherein the at least one processor is configured to receivethe at least two different sets of images of the sporting implement fromthe at least two cameras, identify at least one motion region in the atleast two different sets of images, identify at least onetwo-dimensional (2D) candidate location of at least one portion of thesporting implement within the at least one motion region, identify athree-dimensional (3D) probable location of the at least one portion ofthe sporting implement based on the at least one 2D candidate location,and approximate a 3D trajectory of the sporting implement based on amultiplicity of probable locations of the at least one portion of thesporting implement identified for a multiplicity of instants.
 18. Thesystem of claim 17, wherein the 3D trajectory of the sporting implementcomprises a path and an orientation of the sporting implement.
 19. Thesystem of claim 17, wherein the sporting implement is a handheldsporting implement, and wherein the at least one portion of the sportingimplement comprises a distal end of the handheld sporting implement. 20.The system of claim 17, wherein the at least one processor is furtherconfigured to superimpose a graphical representation of the 3Dtrajectory of the sporting implement over at least one of the at leasttwo different sets of images.